One problem in the field of airborne radars is performing a number of simultaneous radar functions requiring the implementation of different waveforms with just one and the same antenna. Among these radar functions, air/air detection, air/ground detection and ground imaging can for example be cited.
When an air/ground function is being carried out, such as, for example, a detection of moving targets on the ground (or GMTI, which stands for Ground Moving Target Indicator), the radar is continuously scanning the ground; thus, the air surveillance cannot be assured and tracking operations are interrupted which can compromise a mission, particularly if a single carrier is employed for the performance thereof. The same problem arises in the case of SAR (Synthetic Aperture Radar) imaging.
In the case of SAR imaging, the illumination and image formation time can typically last from one to several tens of seconds, even a minute depending on the desired resolution on the image, on the image capture range, on the ground observation angle, on the speed of movement of the carrier. As an example, if a resolution of 30 cm is wanted at a distance of 60 km, the required angular resolution is 5 μrad. With a carrier moving at a speed of 240 m/s and observing the ground with an angle of 45°, the illumination time for a wavelength of 3 cm is then approximately 18 s.
The interruption of the aerial tracking operations for a number of tens of seconds, even longer than a minute, can be problematical. Despite this interruption, the status parameters of the targets could be extrapolated so as to rapidly reacquire the targets at the end of the time allotted for the SAR, assuming that they are moving in a straight line. For maneuvering targets, the extrapolations will generally give results that do not make it possible to resume the tracking operations after such a long interruption.
Also, the tactical situation can change with new targets which are not detected since, in addition to the current tracking operations, the surveillance is also interrupted during the SAR image capture which can constitute a significant handicap during the mission.
The color transmission techniques associated with an array of antennas with multiple input/output ports (MIMO, Multiple Input Multiple Output) are techniques that can make it possible to perform a number of radar functions simultaneously. The principles of color transmission are notably described in a paper by François Le Chevalier: “Space-time transmission and coding for airborne radars” published in Radar Science and Technology, volume 6, December 2008. These color transmission radars make it possible to use new beam-forming techniques and new waveforms, in which different signals are transmitted simultaneously in different directions to jointly code the space and time. These signals are then processed coherently on reception. These new techniques make it possible to optimize the overall performance levels of the radar for a given mission, while being more robust to electromagnetic counter-measures. These techniques make it possible, among other things, to increase the instantaneous angular coverage of a radar in exchange, for example, for a larger radar bandwidth or more numerous radar ambiguities. These techniques use a generalization of the antennas with subarrays because they make use of a number of collocated transmission channels in addition to the reception channels.
In an airborne context, the number of subarrays will necessarily remain limited to the order to ten or so, for costs and bulk reasons. The maximum widening of the instantaneous angular coverage will therefore remain limited to a factor √10≈3 in both vertical and horizontal directions, that is to say approximately 10° for the usual wavelengths and antenna sizes. Now, in the case of SAR imaging for example, it is obligatory to illuminate the ground with an angle of a number of tens of degrees, this angle being able to be constant during the illumination time (case of the “StripMap” mode) or else locked so as to illuminate a fixed zone on the ground (case of the “SpotSar” mode). Referring to FIG. 1, the aerial angular field to be covered is approximately 120° in the horizontal plane and up to 15° in elevation (i.e. 0.55 sr). Consequently, the widening of the angular coverage is insufficient to illuminate the combination of the fields associated with the air-air and air-ground functions.
Furthermore, the different variants of the color transmission techniques demand the use of waveforms with pulse repetition intervals common to the transmission channels which involves revising the design and the development of the current modes.
Another drawback lies in the fact that the transmission array simultaneously covers a wide field in elevation. This means powering each of the feeds of the transmission array with a coded signal, the codes having to be all mutually orthogonal. The necessary electronics and the associated processing are therefore complex, and all the more so when the size of the array is significant.
A multifunction radar is known from the prior art, notably in the publication by J.-L. Milin et al. “AMSAR—A European Success Story in AESA Radar”, Proc. International Radar Conference, October 2009. In this device, the antenna is structured in subarrays consisting of a subset of the radiating elements distributed over the total surface area of the antenna. The antenna transmits by a sum channel and the subarrays, in reception, make it possible to apply space-time processing operations in order to reduce the nuisance caused by the spurious ground echoes.
One drawback with this structure is that it has only one transmission channel, so it does not therefore make it possible to transmit a number of different waveforms simultaneously.
One way of assuring an aerial surveillance while a “ground” function (GMTI or SAR) is being carried out consists in interleaving the two “air” and “ground” functions. This technique is known by the term “interleaved scanning” or long-term color transmission. In this type of transmission, the pulses are transmitted successively in different directions, possibly with an additional phase or frequency coding per pulse.
This transmission concept does indeed allow for a wider quasi-instantaneous angular coverage; however, it does introduce additional speed ambiguities in each direction explored.
Furthermore, when applied to the issue of the simultaneous performance of “air” and “ground” functions such as, for example, target detection or SAR imaging, the interleaved scanning compromises the hitherto qualified waveforms. In effect, the air/ground waveforms have low recurrence frequency so as not to have range ambiguities; now, the air/air functions support this type of waveform badly, notably because of the speed ambiguities of the fast targets. It is therefore necessary to provide the waveforms with additional properties to cancel or reduce the speed ambiguity effect.
The use of the interleaved scanning concept to simultaneously perform a number of radar functions therefore requires significant modifications to the structure of the radars, with complete requalification of the waveforms. The consequence is a relatively long-term installation and a very high cost.